Multinuclear NMR spectroscopic studies of aryltrimethylsilanes and aryldimethylphosphaneboranes
Abstract
Proton, boron-11, carbon-13, silicon-29, and phosphorus-31 NMR chemical shifts and coupling constants are reported for nine ortho- and 2,6-disubstituted aryltrimethylsilanes and five similarly substituted aryldimethylphosphaneboranes. Resonances in the natural-abundance carbon-13 NMR spectra for both sets of derivatives are assigned on the basis of additivity relationships, proton-coupled spectra, and relative magnitudes of | J( P C) | coupling constants. Carbon-13 chemical shifts and | J( C H) | coupling constants indicate that the P(BH )(CH ) group is electron-withdrawing. The C chemical shifts of aryl C(5) carbons can be attributed to steric inhibition of resonance of about the same magnitude as that produced by ortho-Si(CH ) . Chemical shift and coupling constant data from previous work are expanded in terms of Taft's dual substituent constants σ and σ . Least squares solutions of these equations for aryldimethylphosphaneborane derivatives provide values of 0.41 for σ and 0.04 for σ for the P(BH )(CH ) group. These constants produce reasonable agreement with observed C chemical shifts and coupling constants in the ortho derivatives. © 1989. 31 13 1 13 1 13 0 0 13 3 3 2 3 3 1 R 1 R 3 3 2